The present invention relates to a process for preparing 4-acetoxy-3-hydroxyethylazetidin-2-one derivatives which have a hydroxyethyl group, wherein the hydroxyl group is protected at the C3-position and has an acetoxy group at the C4-position. It is known that 4-acetoxy-3-hydroxyethylazetidin-2-one derivatives are useful intermediates for preparing carbapenem .beta.-lactam antibiotics such as thienamycin and penem .beta.-lactam antibiotics (cf., for example, Tetrahedron Letters by Reider et al., vol. 23, page 2293, 1982 and Chem. Pharm. Bull. by Yoshida et al., vol. 29, page 2899, 1981).
There hitherto have been known processes for synthesizing 4-acetoxy-3-hydroxyethylazetidin-2-one derivatives, for instances, synthesis from 6-aminopenicillanic acid (cf. Chem. Pharm. Bull. by Yoshida et al., vol. 29, page 2899, 1981), synthesis from threonine (cf. Tetrahedron by Shiozaki et al, vol. 39, page 2399, 1983) synthesis from aspartic acid (cf. Tetrahedron Letters by Reider et al., vol. 23, page 2293, 1982) and synthesis from a metal enolate of .beta.-hydroxy butyric acid (cf. Chemistry Letters by Nakai et al., page 1927, 1984). However, these processes have a problem that industrially unfavourable reagents such as mercury compound, e.g., mercury acetate or mercury sulfate and lead tetraacetate are employed in order to introduce an acetoxy group at the C4-position of the .beta.-lactam ring.
The inventors found a process for introducing an acetoxy group at the C4-position by using an N-protected .beta.-lactam compound having an O-protected hydroxyethyl group at the C3-position and a silylether group at the C4-position, and filed a patent application (cf. Japanese Unexamined Patent Publication No. 18758/1986).
However, this process needs two steps, one is to introduce previously a protective group for N of the .beta.-lactam and the other is to remove the protective group after the acetoxy group is introduced at the C4-position.